High octane gasoline is required for modern gasoline engines. Formerly it was common to accomplish octane number improvement by the use of various lead-containing additives. As lead was phased out of gasoline for environmental reasons, octane ratings were maintained with other aromatic and low vapor pressure hydrocarbons. Environmental damage caused by the vaporization of low vapor pressure hydrocarbons and the health hazards of benzene in motor fuel will lead to further restrictions on octane blending components. Therefore, it has become increasingly necessary to rearrange the structure of the C5 and C6 hydrocarbons used in gasoline blending in order to obtain high octane levels. Catalytic isomerization is a widely used process for this upgrading.
The traditional gasoline blending pool normally includes C4 and heavier hydrocarbons having boiling points of less than 205° C. (395° F.) at atmospheric pressure. This range of hydrocarbon includes C4 to C6 paraffins and especially the C5 and C6 normal paraffins which have relatively low octane numbers. The C4 to C6 hydrocarbons have the greatest susceptibility to octane improvement by lead addition and were formerly upgraded in this manner. With eventual phase out of lead additives octane improvement was obtained by using isomerization to rearrange the structure of the paraffinic hydrocarbons into branched-chain paraffins or reforming to convert the C6 and heavier hydrocarbons to aromatic compounds. Normal C5 hydrocarbons are not readily converted into aromatics, therefore, the common practice has been to isomerize these lighter hydrocarbons into corresponding branched-chain isoparaffins. Although the C6 and heavier hydrocarbons can be upgraded into aromatics through hydrocyclization, the conversion of C6's to aromatics creates higher density species and increases gas yields with both effects leading to a reduction in liquid volume yields. Moreover, the health concerns related to benzene are likely to generate overall restrictions on benzene and possibly aromatics as well, which some view as precursors for benzene tail pipe emissions. Therefore, it is preferred to change the C6 paraffins to an isomerization unit to obtain C6 isoparaffin hydrocarbons. Consequently, octane upgrading commonly uses isomerization to convert C6 and lighter boiling hydrocarbons.
The effluent from an isomerization reaction zone will contain a mixture of more highly branched and less highly branched paraffins, hydrogen and light ends containing from one to four carbon atoms. Hydrogen is removed in a product separator unit and recycled to the isomerization reactor. The light ends are removed from the desired isomerized products in a stabilizer. In order to further increase the octane of the products from the isomerization zone, normal paraffins, and sometimes less highly branched isoparaffins, are typically recycled to the isomerization zone along with the feedstream in order to increase the ratio of less highly branched paraffins to more highly branched paraffins entering the isomerization zone. A variety of methods are known to treat the effluent from the isomerization zone for the recovery of normal paraffins and monomethyl-branched isoparaffins for recycling these less highly branched paraffins to the isomerization zone.
Relatively higher octane isomers are commonly separated from lower octane normal paraffins and monomethyl-branched paraffins by using a distillation zone, adsorptive separation or some combination thereof. General arrangements for the separation and recycling of C5 and C6 hydrocarbons in isomerization units are shown and described at pages 549 through 5-51 of THE HANDBOOK OF PETROLEUM REFINING PROCESSES, edited by Robert A. Meyers, published by McGraw Hill Book Company (1986). Distillation is a primary method of recovering the desired higher octane isomers from the lower octane isomers with the lower octane isomers potentially being recycled to the isomerization zone.
Similarly, another technique for separating the high octane isomers from the lower octane isomers is adsorptive separation under liquid phase conditions. In such methods, the isomerization effluent contacts a solid adsorbent having a selectivity for normal paraffins to effect the selective adsorption of normal paraffins and allow recovery of the isoparaffins as a high octane product. Contacting the normal paraffin containing adsorbent with the desorbent material in a desorption step removes normal paraffins from the adsorbent for recycle to the isomerization zone. Both the isoparaffin and normal paraffin containing streams undergo a separation for the recovery of desorbent before the isoparaffins are recovered as a product and the normal paraffins recycled to the isomerization zone. Liquid phase adsorption has been carried out in conventional swing bed systems as shown in U.S. Pat. No. 2,966,528. The use of simulated moving bed systems for the selective adsorption of normal paraffins is also known and disclosed by U.S. Pat. No. 3,755,144. Simulated moving bed systems have the advantage of increasing recovery and purity of the adsorbed and non-adsorbed components in the isomerization zone effluent for a given unit of adsorbent material.
Adsorption processes using vapor phase adsorption for the separation of normal and branched paraffins are also well known. Examples of such processes are described in U.S. Pat. No. 4,709,116 and U.S. Pat. No. 4,709,117. These references teach the use of multiple adsorbent vessels and the steps of adsorbing and desorbing the normal paraffins from an isomerization zone effluent. In addition, one or more steps of blowdown or void space purging are also taught to increase the recovery of product hydrocarbons.
Recent efforts in adsorptive separation teach adsorbents and flow schemes for also separating monomethyl paraffins from dimethyl-branched paraffins. U.S. Pat. No. 4,717,784 and U.S. Pat. No. 4,804,802 disclose processes for the isomerization of a hydrocarbon feed and the use of multiple adsorptive separations to generate normal paraffin and monomethyl-branched paraffin recycle streams. In such systems the effluent from the isomerization zone enters a molecular sieve separation zone that contains a 5 A-type sieve and a ferrierite-type sieve that adsorb normal paraffins and monomethyl-branched paraffins, respectively. U.S. Pat. No. 4,804,802 discloses steam or hydrogen as the desorbent for desorbing the normal paraffins and monomethyl-branched paraffins from the adsorption section and teaches that steam or hydrogen may be recycled with the normal paraffins or monomethyl-branched paraffins to the isomerization zone.
Another method of recovering the high octane isomers from lower octane isomers and normal paraffins uses adsorptive separation followed by distillation. U.S. Pat. No. 3,755,144 shows a process for the isomerization of a pentane/hexane feed and the separation of normal paraffins from the isomerization zone effluent. The isomerization zone effluent is separated by a molecular sieve separation zone that includes facilities for the recovery of desorbent from the normal paraffin containing stream that is recycled to the isomerization zone. An extract stream that contains isoparaffins is sent to a deisohexanizer column that separates isopentane and dimethylbutane as a product stream and provides a recycle stream of isohexane that is returned to the isomerization zone. U.S. Pat. No. 5,043,525 teaches eliminating the fractionation columns typically used to separate desorbent from streams exiting an adsorptive separation zone. U.S. Pat. No. 5,602,291 teaches using pressure swing adsorption to separate isomerization effluent. U.S. Pat. No. 2,966,528, discloses a process for the isomerization of C6 hydrocarbons and the adsorptive separation of normal hydrocarbons from branched-chain hydrocarbons.
The present invention involves an isomerization process that eliminates a commonly employed unit. Specifically, the present invention eliminates the need for a stabilizer column to separate the light ends from the desired isomerized products. Stabilizer columns have been traditionally used in isomerization processes, see U.S. Pat. No. 5,026,951, U.S. Pat. No. 5,146,037, U.S. Pat. No. 4,831,209, U.S. Pat. No. 5,705,730 and U.S. Pat. No. 5,326,926. In the present invention however, the effluent of the isomerization reactor is passed to a product separator to remove and recycle hydrogen, and then to an adsorptive separation zone. In the separation zone unadsorbed hydrocarbons are recovered as high octane product, and straight-chain hydrocarbons having a lower octane value are desorbed using a normal paraffin desorbent. The straight-chain hydrocarbons are recycled to the isomerization zone. The costly stabilizer column is eliminated and the function of the stabilizer is accomplished by the flash drum and the separation zone of the invention. The present invention provides an isomerization process having lower capital costs and lower utilities costs due to the elimination of the stabilizer column.